Autumn is one of those magical times of year when you suddenly get lucky for a week or two to see the bursting change of colors. Especially nice when as you change elevation you can see the color change happening in different phases.
While the US is probably one of the best places to see the foliage change, I would love to time a visit to Japan just right to see the colors change. When I visited Kyoto a few years back, it seemed like I was just a week or so early. I could only imagine how beautiful it would be a few days later.